Large multilamellar liposomes, made up of phospholipids and cholesterol, containing gangliosides and glycoprotein components of the receptor for thyrotropin (TSH) undergo permeability changes when challenged with TSH. These changes, which are specific for TSH, may resemble comparable perturbations, caused by the hormone on authentic thyroid cells, responsible for hyperpolarization of the plasma membrane and modifications of ion fluxes. This system will be used in studies aimed at the identification of the role of membrane components on the expression of receptor activity. Special attention will be focused on the role of phospholipids and glycolipids. Rat thyroid cell lines can be grown "in vitro" under conditions suitable for the maintainance of differentiated functions (e.g. thyroglobulin biosynthesis). These calls can be infected with and transformed by RNA tumor viruses and, in this case, they undergo major morphological and biochemical changes. These "transformed" thyroid cells will be used in studies in which abnormalities of differentiated cell functions will be correlated with alterations of plasma membrane compositions, with special attention focused on the structure and the function of the receptors for TSH.